Superior UV photodetector performance of TiO₂ films using Nb doping

Highlights

• Effect of Nb-doping toward tailoring the photosensitivity of TiO₂ films has been elucidated.

• Charge compensation due to Nb-doping in TiO₂ crystal lattice has been provided.

• A higher photocurrent has been achieved using 1.0 at% Nb-doped TiO₂ film.

• A high external quantum efficiency has been attained even under a low power density of UV-illumination.

Abstract

Titanium dioxide (TiO₂) is one of the excellent materials used for photodetector applications due to its high photoactive nature. In this research, highly sensitive ultra-violet (UV) photodetectors with high detectivity and external quantum efficiency (EQE) are developed by depositing Nb-doped TiO₂ (TNO) films onto a Si/SiO₂ substrate at different Nb concentrations. The doping effect of Nb on the electronic structure and photodetector performance of TiO₂ films has been investigated. The charge compensation mechanism due to Nb-doping has been explained using X-ray photoelectron spectroscopy. Lower doping concentrations of Nb⁵⁺ result in creating a Ti vacancy and an excess electron per Nb atom, contributing to the enhanced photocurrent. The metal-semiconductor-metal (M-S-M) structure of the device generates an ohmic-contact and the Si/SiO₂ layer assisted toward reducing the surface recombination. All these factors cause an enhancement in the photocurrent, detectivity, and EQE of the TNO photodetector. Finally, the optimized TNO film grown using 1.0 at.% Nb shows improved photodetector properties and is suitable for ultra-sensitive UV photodetector applications.

Introduction

Direct conversion of ultra-violet (UV) light energy into an electrical signal is a promising strategy for the construction of UV photodetectors [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Therefore, focus on the research and development of photodetector devices with enhanced sensitivity and speed is essential. Subsequently, photodetectors have found applications in a vast number of fields, such as laser welding, gas sensing, biomedical imaging, microelectronics, pollution monitoring, photoelectrodes, and defence devices [10,11]. Due to ozone-layer depletion, harmful UV radiation is penetrating our atmosphere. Exposure to such harmful UV-radiation is one of the prime factors that cause skin cancer and other diseases [12]. In addition, the amount of radiation exposed onto plants during vertical farming is required to continuously monitor and control growth to achieve improved yields. These factors urge the need for extensive research on developing UV photodetectors. Until now, very few suitable materials, such as ZnO, WO₃, MoO₃, TiO₂, NiO, and Ga₂O₃, have been utilized in UV photodetectors [[13], [14], [15], [16], [17], [18]]. Transition metal oxide semiconductors have been studied over the past few years due to their abundance and chemical stability [19,20]. They have also found applications in electronics, optics, energy materials, photonic sensors, and lasers [[21], [22], [23], [24], [25], [26]]. Traditional semiconductor-based photodetectors, such as Si, Ge, and GaAs, have a narrow band gap, which absorbs a wide range of wavelengths in the electromagnetic spectrum, making them inappropriate for UV-photodetection. However, transition metal oxides have a wide band gap ranging from 2.2 to 4.0 eV, which can absorb UV-radiation and convert it into an electrical signal.

Among the transitional metal oxides reported to date, TiO₂-based thin films have been extensively investigated for applications such as self-cleaning surfaces, air-water treatment, UV photodetectors, and deodorizers because of their strong oxidation capacity, profusion in nature, and strong photoactivity [[27], [28], [29], [30]]. Among the many ways to vary the band gap and electronic structure of the material to enhance the photoactivity of TiO₂ films under UV illumination, doping could be one of the best solutions. Few reports exist on the TiO₂-based photodetectors and several dopants, such as Co, Fe, Mn, Zn, Er, Nb, and Al, which exhibit the enhanced photoactivity in doped TiO₂ compared to pristine TiO₂ [[31], [32], [33], [34], [35], [36]]. Among these dopants, Nb is expected to effectively enhance the absorption of UV-radiation to achieve superior photodetector performance because pentavalent doping induces high charge carrier concentration and shifts the Fermi energy level towards the conduction band for efficient conversion of light into an electrical signal [37].

The present study aims to enhance the fundamental figures of merit, such as the responsivity, sensitivity, and response time of TiO₂-based photodetectors under UV-illumination. In this work, Nb-doped TiO₂ (TNO) films were deposited onto a Si/SiO₂ substrate using a magnetron sputtering technique at different Nb-doping concentrations. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS) were carried out on the as-grown TNO films to study the structural, morphological, and surface chemical states, respectively. Furthermore, the photodetector parameters were also measured and determined to optimize the Nb-doping concentration in TiO₂ films for active UV photodetector applications.